Rice rhizodeposits affect organic matter priming in paddy soil: The role of N fertilization and plant growth for enzyme activities, CO 2 and CH 4 emissions

Carbon dioxide (CO2) and methane (CH4) production in paddy soils play a crucial role in the global carbon (C) cycle and greenhouse gas emissions. A rhizosphere priming effect (RPE) may change these emissions, but the relationships between RPE, CH4 emission, and the effect of N fertilization are unknown. We investigated the RPE on CO2 and CH4 emissions and their dependence from N fertilization in a13CO2 continuous labelling experiment by partitioning total CO2 and CH4 derived from roots and soil organic matter (SOM). Because of plant-derived CO2, rice plants strongly increased total CO2 emission compared to that from unplanted soil. SOM-derived CO2 and CH4 increased in the presence of roots but decreased after N fertilization. The RPE for CO2 at an early growth stage (≤40 days) was negative: −1.3 and −1.9 mg C day−1 kg−1 soil without and with N fertilization, respectively. However, 52 days after transplanting, RPE for CO2 got to positive. The RPE for CH4 increased gradually up to 1.6 and 0.5 mg C day−1 kg−1 soil at the end of the experiment without and with N fertilization, respectively. Moreover, the RPE for CH4 got half of the RPE for CO2 after 64 days showing the relevance of CH4 emissions for greenhouse gases balance and C cycling in paddy ecosystems. The RPE for CO2 and CH4 emissions increased with microbial biomass content and activities of xylanase and N-acetylglucosaminidase. Supporting the results to RPE, the enzyme activities decreased with N fertilization, suggesting that reduced N limitation decreased microbial potential to mine N from SOM. In conclusion, for the first time we showed that root-microbial interactions stimulated SOM mineralization in rice paddies through rhizosphere priming effects not only for CO2 but also for CH4, but the RPE decreased with N fertilization.